The present invention relates to an improved adaptive optics control system and in particular, to a signal-to-noise ratio-tuned wavefront corrector for use therein.
Optical wavefronts are subject to distortion when passed through certain mediums. These distortions can degrade the quality of images of an object being observed through an image-forming device such as a camera or telescope. Such distortions and degradations can be especially severe and problematic in turbulent mediums such as the atmosphere. Some of the diffractive effects of such turbulence include image blurring caused by the turbulence aberrations, scintillation caused by propagation through strong turbulence, and speckle caused by coherent scattering from diffuse objects.
Various adaptive optic control systems, mechanisms and methods have been developed to try to correct for the blurring that results from such distorted wavefronts. One such control system for use with telescopes, often referred to as a “tip-tilt” correction, involves tilting the secondary mirror of the telescope several times a second to reduce or eliminate the dancing motion of the image. This method, however, only provides a small improvement in the sharpness of the image in applications involving large telescopes.
Various other adaptive optics control systems have been developed to try to better compensate for such distortions. Examples of various such systems are disclosed in “Adaptive Optics for Astronomical Telescopes,” by John W. Hardy, 1998 (pages 31–33, and 55–69), which is incorporated herein by reference. Such control systems typically include a wavefront slope sensor for measuring the phase differences or phase slopes between points of a wavefront, a wavefront reconstructor for estimating the wavefront phase from the phase differences, a control system for reducing the effects of noise, and a wavefront corrector for correcting the wavefront based thereon. The wavefront sensor is usually in the form of a Hartmann wavefront sensor as shown in FIG. 2. Hartmann wavefront sensors use an array of lenslets or a mask pierced with holes for dividing the distorted wavefront into an array of subapertures. Each of the beams in the subapertures is focused onto one or more detectors disposed behind the holes or lenslets. When the distorted wavefront passes through the holes or lenslets, it forms an array of spots on the detectors, which are indicative of the local wavefront slope or tilt, if any, in the corresponding subaperture. Typically, the wavefront slope sensor includes an analog-to-digital converter and one or more processors to compute the wavefront slopes.
The wavefront corrector is usually in the form of a deformable mirror which compensates for such distortions. Deformable mirrors typically comprise a face plate to the back of which a plurality of actuators are secured. The actuators expand or contract in length upon application of a voltage or a magnetic field in accordance with the electrical commands generated by the wavefront reconstructor, thereby pushing or pulling on the faceplate and causing the mirror to change its shape to make the appropriate corrections to the distorted waveform. The actuators are typically arranged in a square or hexagonal array defining a plurality of zones, and are capable of displacing the faceplate locally within each zone by a few micrometers up or down.
Current wavefront slope sensors, however, are subject to various measurement errors that degrade wavefront correction. First, there is noise in each measurement, including read-out noise and dark-current noise in the detector and shot noise in the received light. Second, diffractive cross-talk can be produced among adjacent subapertures. Third, the subapertures and detectors may be misaligned. Fourth, partially filled lenslets can cause stretching of the spot size on the detectors. Fifth, the intensity across each lenslet may not be uniform, which can also change the spot sizes and shapes. In addition, Hartmann wavefront sensors are unable to provide accurate wavefront phase slope measurements in some subapertures during conditions of severe turbulence due to large variations in the intensity and phase within a subaperture. One approach to minimize such effects is to combine a Hartmann sensor with another device, such as a unit shear lateral shearing interferometer wavefront sensor as is disclosed in U.S. Pat. No. 4,518,854, which is incorporated herein by reference.
Further, conventional control systems assume that all of the subaperture-based measurements are of equal quality and that all of the computed signals for the wavefront corrector actuators are of equal quality. However, the measurements in the edge subapertures (i.e., around the outer aperture, around interior obscurations, or in the vicinity of struts) are typically only partially filled, and so have lower signal-to-noise ratios and are of a lesser quality than the measurements in the interior subapertures. The wavefront correction signals for edge actuators are also typically of a lesser quality than those corresponding to non-edge actuators, due in part to the fact that the quantity of data in the vicinity edge actuators is limited and due to the fact that the estimate of the phase for each actuator is dominated by the data in the vicinity of the corresponding subaperture. Therefore, the correction signals for such edge actuators can be extremely noisy, making the system unstable. As a result, the overall bandwidth of such control systems is often reduced to the level of the least stable actuator estimate. Such a limitation on the bandwidth, however, can significantly degrade the performance of the system.
As a result, there is a need for an improved, adaptive optics control system which overcomes these problems.